This invention relates to a microstrip antenna, and especially to a 4 resonance microstrip antenna which can be used at four frequencies. This invention also relates to a polarized wave microstrip antenna which enables polarization with only a single electric supply point.
A microstrip antenna has two conductive plates on both sides of a dielectric plate and uses a radiational loss of an open level resonance circuit Microstrip antennas are particularly useful, because a microstrip antenna is:
(1) low profile PA1 (2) light weight and compact and PA1 (3) easy to make.
A microstrip antenna has a narrow band width characteristic and works in a special frequency band.
A radio communication system is known which uses waves of many different frequencies. A spectrum diffusion communication system which uses frequency hopping is one such radio communication system. In this system, a signal is diffused directly by a modulation frequency (transmitting frequency) which is hopped by a code series. This requires a radio station which consists of a system having radio transmitting and receiving power at many frequencies.
A microstrip antenna is suitable for a place where there is a limited space for instance a car, in the case of a radio communication using only one frequency, for the above-mentioned reasons. In the case of radio communication using several frequencies such as spectrum diffusion communication by frequency hopping, however, a microstrip antenna is not suitable for a place where there is a restricted space, because a microstrip antenna has a narrow bandwidth characteristic, so that as many elements (radiation conductive elements) are necessary as there are frequency bands.
For example Japanese Laid-Open Patent Application No. 56(1981)-141605 shows a 2 resonance microstrip antenna having an elliptic element which is excited in both a long axis mode and a short axis mode. It is possible to use this antenna in two frequency bands in order to obtain an intersection excitement mode by providing an electric supply point at an intersection of the long axis and the short axis of the ellipse.
Japanese Laid-Open Patent Application No. 59(1984)-126304 shows a 2 resonance microstrip antenna which has two half sized elements and uses an electric image effect. The element size is cut by about half by using an electric image effect which is made by being short at a position where a current distribution of the element is zero so that the 2 frequency microstrip antenna can be made by connecting two half sized elements which resonate at different frequencies, at the short point.
However, the above-mentioned 2 resonance microstrip antenna cannot be used at several frequencies because of its constitution. The latter has two half sized elements separately which resonate at different frequencies, although it has a common short point. There is no reduction of number of elements when the antenna is used at two frequencies. Polarization is either straight line or circular (including an ellipse) according to the shape of the element (radiation conductive element). the electric supply point, and the method of electric supply.
Several frequencies are used at the same time for radio communication, for example in a transmission and reception circuit. In this case, it is necessary to isolate each frequency in order to prevent adverse effects including interference If a polarized wave microstrip antenna were used for this kind of a radio communication system, there would have to be a special polarized wave microstrip antenna for each frequency, because a polarized wave microstrip antenna has a narrow bandwidth characteristic. This means that the microstrip antenna would become large in size, thus losing one advantage of microstrip antennas (light weight and compactness).
Many combinations of polarized wave microstrip antennas have been devised in an attempt to solve this problem. For example, Japanese Laid-Open Patent Application No. 57(1982)-91003 shows a 2 frequency polarized wave microstrip antenna which uses a quadrangle combination of triangle polarized wave microstrip antennas for transmission and reception respectively. It is possible to minimize an increase of space taken by a 2 frequency polarized wave antenna in the former. However, other advantages of the microstrip antenna, such as its low profile, light weight, compactness and ease of manufacture are lost by having such a multistage configuration. With the latter, the overall space required by the microstrip antenna is decreased, because small triangle polarized wave microstrip antennas are used for transmission and reception. However, the space required is greater than that required for a single microstrip antenna.